Esta foi uma questão de discussão sobre o Doctor Who Wikia ;
Em suma, Timelords parecem ser capazes de quase se reconhecerem imediatamente. Além disso, eles são capazes de telepatia, de modo que qualquer dúvida persistente poderia ser facilmente apagada pela condução de um contato mental;
They can still recognise each other regardless of regeneration. This was shown in The Armageddon Factor when Drax was able to recognise the Fourth Doctor and The Twin Dilemma when Azmeal was able to recognise the Sixth Doctor, despite their last encounter being in the Doctor's fourth incarnation.
Given that Time Lords have been shown to have some telepathic ability, it is reasonable to suppose that this is how they recognise each other after regenerations. However, this doesn't explain how the Doctor recognised the Master on television in "The Sound of Drums". It is possible, therefore, that certain other traits are carried from one incarnation to the next e.g. Subtle facial characteristics.